\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
x_{n+1}:=N_f(x_n)=x_n-\frac{1-a/x_n^2}{2a/x_n^3}=\frac{x_n}{2}
\end{equation*}
\end{document}
